As always in the case of cosmic rays, the information we can gather experimentally concerns: energy spectrum, mass composition and anisotropy. Since the first observation of an EAS with energy \(~10^{20}\)  eV back in 1962 [1]}, the experimental study of UHECR clarified several important facts (see [2]}, [3]} and references therein): (i) UHECR are charged particles, with a limit on photon and neutrino fluxes at \(10^{19}\)  eV at the level of few percent and well below respectively, (ii) the spectrum observed at the Earth shows a slight flattening at energies around \(5\times 10^{18}\)  eV (called the ankle) with (iii) an instep at \(10^{19}\)  eV and (iv) a steep suppression at the highest energies, (v) mass composition is dominated by light particles (proton and helium) at energies around \(10^{18}\)  eV becoming progressively heavier starting from energies around \(3\times 10^{18}\)  eV toward the highest energies.
